1. Field of the Invention
The present invention relates to a method for reconstructing a shape of a subject and an apparatus suitable for implementing this method.
2. Description of the Related Art
As an example of a method for detecting surface inclinations of a subject in pixel units and reconstructing a shape of the subject by integrating the detected inclinations, there is a method disclosed in reference I "THE TRANSACTIONS OF THE INSTITUTE OF ELECTRONICS, INFORMATION, AND COMMUNICATION ENGINEERS OF JAPAN, PART D-II Vol. J77-D-II No. 9 pp.1797-1805, September 1994". This method obtains plural images of a subject by photographing said subject from a fixed position as changing only the direction of a light source. Next, it detects surface inclinations of said subject in pixel units from pairs of luminances of arbitrary pixels corresponding to each other in these images. And it reconstructs a shape by integrating the detected inclinations (see "lines 1 to 9 in the left column of p. 1798 of the reference I"). When performing this integration, it is necessary to determine integral intervals. This is for the purpose of distinguishing a subject as an object of photographing from its background.
As a method for determining an integral interval, there have been up to now (a) a method of separating an integral interval from an image by means of a two-level process or the like (see "Line 5 from the bottom of p. 1799 of the reference I") and (b) a method of identifying an integral interval by comparing a subject with a reference object on the basis of a reflectivity map and estimating a domain different in reflectivity from an estimated error value of a pixel normal vector (see "Line 4 of p. 1799 to line 1 of p. 1800 of the reference I").
And for performing the integration, a method has been adopted (1) which uses the brightest point in an image as an initial value for the integration (see "Lines 9 and 10 of p. 1800 of reference I") or (2) which performs integration after obtaining an integration starting point to minimize an error of an initial inclination value (see reference II "THE TRANSACTIONS OF THE INSTITUTE OF ELECTRONICS, INFORMATION, AND COMMUNICATION ENGINEERS OF JAPAN", Part D-II Vol. J76-D-II No. 10, line 28 or later in the left column of p. 2244).
And in order to suppress accumulation of integral errors when performing the method (2), a method is also adopted which computes average values of depth as weighting with estimated errors (see "Lines 19 and 20 in the left column of p. 1800 of reference I"). And in order to reduce an integral error, a method is also adopted which selects an integral path (see "Line 3 or later from the bottom of the left column of p. 2244 of reference II").
However, in case of performing a two-level process for determining an integral interval, there is a high possibility that domains different in color on a subject having various colors are treated as different integral intervals. And furthermore, in case that an edge of the subject cannot be successfully caught when performing a two-level process, since an error happens in determining an integral interval and the error is superposed on the integration, reconstruction of a shape cannot be successfully performed. In case of adopting a process of using a reflectivity map when determining an integral interval, it is laborsome to prepare the reflectivity map. And in case of adopting any of a process of obtaining the brightest point as an integration starting point and a process of obtaining an integration starting point to minimize an error of an initial inclination value, the process is very laborsome. And these laborsome processes require an apparatus of complicated composition. In case of adopting a method of weighting with estimated errors in order to suppress accumulation of integral errors, there is a possibility that an integral distortion is caused by suppression of the estimated errors. And it is not easy to select an integral path capable of reducing an integral error, and integrated values are made different depending upon the selected paths.
It is therefore an object of the invention is to provide a new method for making it easier to determine an integral interval required for reconstructing a shape, and an apparatus suitable for implementing the method.